Effect of Temperature on Reaction Rate (AQA A-Level Chemistry): Revision Notes

1.7.4 Effect of Temperature on Reaction Rate

Temperature plays a critical role in determining the rate of reaction. When the temperature of a reaction mixture is increased, the kinetic energy of the particles increases, leading to more frequent and energetic collisions between reactant molecules.

How Temperature Affects Molecular Energies

Increasing the temperature causes a shift in the Maxwell-Boltzmann distribution of molecular energies:

• At a higher temperature, the distribution curve shifts to the right and the peak becomes lower. This indicates that more molecules have higher kinetic energy.
• At a lower temperature, the curve shifts to the left and the peak becomes higher. This shows that most molecules have lower kinetic energy.

Important Features of the Maxwell-Boltzmann Distribution at Different Temperatures

1. Higher temperatures:

• The distribution is broader and flatter, meaning more particles have higher energy.
• A larger number of molecules have energies greater than the activation energy, ($E_a$), which is required for a reaction to occur.

2. Lower temperatures:

• The distribution is narrower and the peak is higher, meaning most particles have lower energy.
• Fewer molecules have enough energy to surpass the activation energy.

Why Does Temperature Increase Reaction Rate?

A small increase in temperature can lead to a large increase in the rate of reaction. This occurs because:

3. Increased kinetic energy: As temperature rises, particles move faster and collide more frequently.
4. More successful collisions: A greater proportion of particles have energy equal to or greater than the activation energy, resulting in more successful collisions per unit of time.

Maxwell-Boltzmann Distribution Explanation

The area under the curve beyond the activation energy threshold ($E_a$) represents the number of molecules that can successfully react. At higher temperatures, this area increases significantly, meaning a larger proportion of particles have enough energy to undergo a reaction.

Thus, even a small increase in temperature leads to a large increase in reaction rate because the number of molecules with sufficient energy for a successful reaction increases exponentially.